Without limiting the scope of the present invention, its background will be described with reference to transmitting downhole data to the surface during completion and production, as an example. The principles of the present invention, however, are applicable throughout the utilization of the well including, but not limited to, drilling, logging and testing the well.
In the past, a variety of communication and transmission techniques have been attempted in order to provide real time data from downhole locations to the surface during the completion and the production process. The ability to obtain real time data transmission provides substantial benefits during operations that enable increased control of these processes. Continuous monitoring of downhole conditions allows for a timely response to possible well control problems and improves operational response to problems or potential problems allowing for the optimization of production parameters. For example, monitoring of downhole conditions allows for an immediate response to the production of water or sand.
Multiple types of telemetry systems have been utilized in attempts to provide real time downhole data transmission. For example, systems have utilized pressure pulses, insulated conductors and acoustic waves to telemeter information. Additionally, electromagnetic waves have been used to transmit data between downhole locations and the surface. Electromagnetic waves are produced by inducing an axial current into, for example, the production casing. The electromagnetic waves include an electric field and a magnetic field, formed at right angles to each other. The axial current impressed on the casing is modulated with data causing the electric and magnetic fields to expand and collapse thereby allowing the data to propagate and be intercepted by a receiving system. The receiving system is typically connected to the ground or sea floor where the electromagnetic data is picked up and recorded.
As with any communication system, the intensity of the electromagnetic waves is directly related to the distance of transmission. Consequently, the greater the distance of transmission, the greater the loss of power and hence the weaker the received signal. Typically, downhole electromagnetic telemetry systems must transmit the electromagnetic waves through the earth's strata. In free air, the loss is fairly constant and predictable. When transmitting through the earth's strata, however, the amount of signal received is dependent upon the skin depth (.delta.) of the media through which the electromagnetic waves travel. Skin depth is defined as the distance at which the power from a downhole signal will attenuate by a factor of 8.69 db (approximately seven times decrease from the initial power input), and is primarily dependent upon the frequency (f) of the transmission and the conductivity (.tau.) of the media through which the electromagnetic waves are propagating. For example, at a frequency of 10 Hz, and a conductance of 1 mho/meter (1 ohm-meter), the skin depth would be 159 meters (522 feet). Therefore, for each 522 feet in a consistent 1 mho/meter media, an 8.69 db loss occurs. Skin depth may be calculated using the following equation. EQU Skin Depth=.delta.=1/.sqroot.(.PI.f.mu..tau.)
where:
.PI.=3.1417; PA1 f=frequency (Hz); PA1 .mu.=permeability (4.PI..times.10.sup.6); and PA1 .tau.=conductance (mhos/meter).
As should be apparent, the higher the conductance of the transmission media, the lower the frequency must be to achieve the same transmission distance. Likewise, the lower the frequency, the greater the distance of transmission with the same amount of power.
A typical electromagnetic telemetry system that transmits electromagnetic waves through the earth's strata may successfully propagate through ten (10) skin depths. In the example above, for a skin depth of 522 feet, the total transmission and successful reception depth would be approximately 5,220 feet. Since many, if not most wells are substantially deeper, systems utilizing electromagnetic waves as a means of transmitting real time downhole data typically involve the use of repeaters to receive, clean up and retransmit to the surface or to the next repeater.
Proposed downhole electromagnetic repeaters have been large, expensive, cumbersome devices that typically form a joint in the pipe string. The cost of such devices typically necessitated that the device be retrieved after use. Further, the installation or removal of such devices is time consuming and expensive due to the need for a rig to trip the pipe string into or out of the wellbore.
Therefore, a need has arisen for an economical system that is capable of real time telemetry of data between downhole equipment and surface equipment in a deep or noisy well using electromagnetic waves to carry the information. A need has also arisen for such a system that is easily installed and that uses inexpensive electromagnetic repeaters for the relaying of electromagnetic transmissions which may remain in the wellbore following use.